Russia said on Thursday it was expelling a Bulgarian diplomat in a tit-for-tat move after Sofia asked a Russian diplomat to leave the country over suspicions of espionage.
The Russian foreign ministry said the Bulgarian ambassador in Moscow, Atanas Krastin, had been handed a note informing him that an employee of the mission was declared "persona non grata".
"The step is a retaliatory measure in response to a decision by the Bulgarian authorities to expel a Russian diplomat from the country in October," the foreign ministry said in a statement.
No further details were provided.
In late October, Sofia expelled a Russian diplomat after he ignored a request to leave following a probe which confirmed he had been spying in the EU member country.
The Russian ambassador was told at the time that the diplomat in question, a first secretary at the embassy in Sofia, has 24 hours to leave the country.
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NATO and EU member Bulgaria was previously a Soviet satellite and is now a rare ally of Russia in Europe.
But tensions spiked in September when Bulgarian prosecutors charged a pro-Russian activist, Nikolay Malinov, with espionage and banned his alleged Russian handler from entering Bulgaria.
Malinov, a former lawmaker, heads Rusofili, the largest pro-Russia non-governmental organisation in Bulgaria.
Russian President Vladimir Putin has given a state award to Malinov.